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RE: Soldier Beam Retaining Wall Design Practice

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Donny, thank you for your insight.  

I recently designed a 12 foot soldier beam wall which required 6x lagging below a depth of 8 feet due the seismic load and the triangular soil pressure distribution provided by the geotechnical engineer.  While the results of my calculations (method #1) required 6x lagging, my review of the literature indicated that 4x lagging is suitable up to depths of 25 feet with an 8 foot pile spacing, even in difficult soils. It was because of the large discrepancy between my calcs and the empirical approach that led me to ask the engineering community for their thoughts on the matter. 

Based on your feedback I will work more closely with the geotechnical engineer on future projects.  In addition, I plan on looking into the background of both the FHWA report and Caltrans design method in hopes of resolving the discrepancy.  

Lastly, from the responses I have had it appears that no one uses the FHWA empirical design method - is this correct? 

Thanks to all of you for your input.

Best regards, Phil


Phil Doody
Mesiti-Miller Engineering, Inc.
224 Walnut  Avenue
Santa Cruz, CA 95060-3836
Phone 831.426.3186 x 118
Fax 831.426.6607
  Please consider the environment before printing this e-mail 

-----Original Message-----
From: Donny Harris [mailto:dharris(--nospam--at)] 
Sent: Tuesday, November 11, 2008 11:24 AM
To: Phil Doody
Cc: seaint(--nospam--at)
Subject: RE: Soldier Beam Retaining Wall Design Practice


I don't do a lot but here is my experience on what I have done.

Generally I have used method #1 based on specific lagging design
pressures provided by the project soils engineer.  I believe that most
soils engineers I have worked with have incorporated method #2 into
these values.  The values I have gotten vary per project and soils
engineer as well as distributed load profile; sometimes truncated
uniform load, sometimes arch shaped distributed load and also triangular
distributions have been provided.  When the design pressures have
required a lagging greater than DF-L No. 2 4x with an 8' pile spacing I
have questioned it and on occasion the loading gets reduced or
"clarified" to allow the 4x to calc out.

I have never seen a lagging failure and on site the lagging often looks
like it is not being loaded very much.  Common sizes are 3x and 4x DF-L
No. 2 or HF No. 1 but probably because I do more OSHPD shoring than
commercial 4x DF-L No. 2 is more common for projects I have done lately.

I can't remember for sure but I think there was an OSHPD document or
maybe it was LADBS several years back that allowed a CD factor of 1.25,
regardless that was for the past code.  I have used a CD=1.15 (it is
technically for snow but the duration of 2 months seems more appropriate
than 7 days for roof live load with a CD of 1.25) Sometimes this gets
challenged by the reviewer and a CD of 1.00 is reverted to. 

Donny Harris, SE
Los Angeles, CA

From: Phil Doody [mailto:phil(--nospam--at)] 
Sent: Monday, November 10, 2008 4:01 PM
To: seaint(--nospam--at)
Subject: Soldier Beam Retaining Wall Design Practice


Is there a consensus on the design of wood lagging for soldier beam
retaining walls? Several design methods appear to be in use and each
very different answers.


Basically, there appears to be at least three ways of designing wood
that I have encountered:


#1 Determine the uniformly applied load  on the lagging due to the soil
pressure at a given depth. Select the lagging based on the required
modulus, S = (Moment)/(Allowable bending stress).


#2 Same process as above but instead apply only 60% of the theoretical
uniform load.  The justification for the 0 .6 reduction factor is that
movement causes the lagging to flex outward, and induces a
redistribution of
soil pressure away from the center of the lagging thus reducing the
moment. This method is described in the Caltrans Trenching and Shoring


#3 The third method of sizing lagging is based on FHWA recommendations
contained in Federal highway Administration Report Number
FHWA-RD-75-130.  A
table in this report provides the minimum thickness lagging for various
conditions, soldier pile spacings and excavation depths.  This method
be used if there are surcharges behind the wall.


The most conservative method is the first method and the one which I am
accustomed.  However, I would like to know if others use methods #1 and
and if anyone has observed lagging failures using these methods? 


Thanks for your input, Phil Doody